Q. I’ve heard drinking cranberry juice can help prevent and treat urinary tract infections. Is this true? If so, how does it work?

A. Diane L. McKay, PhD, assistant professor at Tufts University School of Medicine, answers: “Cranberries are a good source of proanthocyanidins (PACs), plant compounds that may inhibit the ability of harmful bacteria to attach to the wall of the urinary tract. The presence of these PACs has led to a lot of research into the role cranberry juice might play in preventing or treating urinary tract infections (UTIs). Results of these studies vary and are largely inconclusive. When you look at all of the research together, there is some evidence that regular consumption of cranberry juice may prevent UTIs among people who have recurrent, uncomplicated infections, but it is not effective at treating UTIs.

“If you suffer with recurrent UTIs and would like to try a daily glass of cranberry juice as a preventive measure, look for products marked ‘100% Cranberry Juice.’ The more readily available ‘Cranberry Juice Cocktail’ is only 27 percent juice, and the rest is added sugars. As you might expect, straight cranberry juice is extremely tart. Try diluting it with another 100 percent juice, such as orange or apple juice, to make it more tolerable. Artificially sweetened cranberry beverages are also an option.

“Other cranberry products, such as cranberry sauce and dried cranberries, are not good choices. A typical serving has a lot of added sugars but not a significant level of PACs. As for cranberry supplements, dose and efficacy varies by product, and it’s hard to know what’s actually in the product you are buying. I strongly recommend sticking with 100 percent cranberry juice so you know exactly what you’re getting, and, of course, seek out appropriate medical treatment if you suspect you have a UTI.”